The present invention relates to a process for producing .alpha.-form gypsum hemihydrate which is large in bulk density and excellent in quality from gypsum dihydrate by a method of "Liquid Methods under Saturated Vapor Pressure".
Different from .beta.-form gypsum hemihydrate, the .alpha.-form gypsum hemihydrate (hereinafter referred to as .alpha.-gypsum) which is excellent in quality can be converted into the high strong molded body of gypsum dihydrate. Accordingly, the .alpha.-gypsum is useful as the material for construction and the raw material for other purposes.
Hitherto, .alpha.-gypsum has been produced by one of the so-called "Wet Processes" from gypsum dihydrate. The representative one of Wet Processes comprises heating a slurry of gypsum dihydrate under a pressure subject gypsum dihydrate to dehydration, thereby converting gypsum dihydrate into .alpha.-gypsum, sujbecting the slurry to solid-liquid separation and drying the thus obtained solid material to obtain .alpha.-gypsum, the process being so-called as "Liquid Methods under Saturated Vapor Pressure".
In the "Liquid Methods under Saturated Vapor Pressure", it is considered indispensable for obtaining .alpha.-gypsum excellent in quality to use an aqueous solution of a substance which has a catalytic activity in crystallization of .alpha.-gypsum (a catalyst for crystallization) as the medium of the reaction of dehydration.
The term ".alpha.-gypsum excellent in quality" means that it contains a smaller mixing-water amount for normal consistency and the strength of the shaped-articles of gypsum dihydrate obtained therefrom is large, and it has been known that the aspect ratio of the crystals of such .alpha.-gypsum is ordinarily small and the bulk density thereof is large.
Hitherto, a large number of substances have been proposed as the catalyst for crystallization. For instance, inorganic salts such as aluminum sulfate, magnesium sulfate, alums, etc. have been known as the catalyst for crystallization (refer to Japanese Patent Publication No. 28-216 (1953) and Japanese Patent Application Laid-Open (KOKAI) No. 55-162426 (1980)), and as the organic catalyst for crystallization, salts and amide-derivatives of higher fatty acids, sulfateesters or higher alcohols, surface active agents having a sulfonic acid group as the hydrophilic atomic group (refer to Japanese Patent Publication No. 31-4375 (1956), and U.S. Pat. No. 4,069,300), water-soluble proteins such as keratin, casein, glues, etc. and salts of lower aliphatic polycarboxylic acids such as succinic acid, citric acid, etc. have been known.
Of the above-mentioned large number of catalysts for crystallization, salts of lower aliphatic polycarboxylic acids represented by salts of succinic acid are considered to be most useful in industry from the viewpoints of the catalytic efficacy, the concentration in use and the price thereof.
Concerning the comparison of the catalystic activity of the above-mentioned salts of lower aliphatic polycarboxylic acids in "Liquid Methods under Saturated Vapor Pressure", M. Sekiya generally reported in the following of "SEKKO and SEKKAI (Gypsum and Lime)", Vol. 51, page 55 (1961). (Refer to FIG. 1).
In addition, of the above-mentioned salts of lower aliphatic polycarboxylic acids, sodium succinate is industrially put to practical use, and also sodium citrate is suitable for that purpose (refer to Japanese Patent Publication No. 57-25495 (1982)).
In view of the above-mentioned technical background, the present inventor studied a polycarboxylate which has a larger catalyst effect for crystallization and is available at a lower price than the above-mentioned polycarboxylate, and as a result, the present inventor has discovered that sulfosuccinate, which is a dicarboxylate, has an excellent catalytic effect for crystallization and on the basis of the discovery, the present inventor has attained the present invention.
By the way, in order to utilize .alpha.-gypsum in an industrially large scale, it is necessary that such an .alpha.-gypsum is excellent in quality and is available at a low price. Namely, though .alpha.-gypsum is far superior to .beta.-gypsum in physical properties, .alpha.-gypsum can be utilized only in an extremely limited field at present because of the high production cost, and the development of an industrially profitable process for producing .alpha.-gypsum in a low cost has been strongly demanded.
In consideration of the above-mentioned situation, various trials for producing .alpha.-gypsum in a low cost comparable to that for producing .beta.-gypsum have been proposed. One of them is the trial for combining the gypsum-byproducing type of desulfurization process of exhaust gases with the production of .alpha.-gypsum.
Such a trial is roughly classified into the following tow processes.
(i) A process wherein sulfur dioxide in an exhaust gas is directly converted into .alpha.-gypsum without being produced gypsum dihydrate (refer to Japanese Patent Application Laid-Open (KOKAI) No. 49-83695 (1974), Japanese Patent Publication No. 57-53292 (1982) and No. 57-49491 (1982)).
(ii) A process wherein after converting sulfur dioxide in an exhaust gas into gypsum dihydrate, the thus formed slurry containing gypsum dihydrate is converted into the slurry containing .alpha.-gypsum (refer to Japanese Patent Application Laid-Open (KOKAI) No. 53-50092 (1978), No. 55-113621 (1980), No. 55-162426 (1980) and No. 56-129611 (1981)).
Originally, the conditions of the liquid composition, which are required for the absorbent liquid of the desulfurization process of exhaust gases, have no relation to the condition of the liquid composition, which are required to the medium for producing .alpha.-gypsum. Neverthless, when a process for producing .alpha.-gypsum is combined directly with a desulfurization process of exhaust gases, the medium for producing gypsum hemihydrate is fundamentally the same as the absorbent liquid in the desulfurization process of exhaust gases, and the adjustments of pH, concentration and dilution of the liquid only can be done.
For producing .alpha.-gypsum excellent in quality, it is indispensable to make a coexistence of a salt of dicarboxylic acid or tricarboxylic acid such as succinic acid, tartaric acid or citric acid as the catalyst for crystallization in the system, and in the absence of such an organic catalyst or crystallization, only .alpha.-gypsum of small bulk density is obtained.
However, in the case where such an organic catalyst for crystallization of carboxylic acids is present in the absorbent liquid for use in the process for removing sulfur dioxide from an exhaust gas (hereinafter referred to as "desulfurization process"), there are inconveniences that the catalyst is decomposed and consumed during oxidation of sulfite, or on occasion, the catalyst inhibits oxidation of sulfite.
Under these circumstances, the process in which the liquid containing an organic catalytic agent for crystallization is used in circulation, as the absorbent liquid in the desulfurization process of exhaust gases and the medium for producing .alpha.-gypsum is only found in Japanese Patent No. 57-53,292/1982.
However, according to the above-mentioned method, as a volatile organic acid such as acetic acid is contained in the liquid, a new problem of vaporization of the acid occurs in the desulfurization process of exhaust gases and at the same time, there is an inconvenience that the whole liquid of the gypsum-formation reaction must be subjected to heat-treatment for the purpose of formation of .alpha.-gypsum, because the process is directly producing .alpha.-gypsum without passing through the step of producing gypsum dihydrate.
Moreover, since every process without passing through gypsum dihydrate produce gypsum at the conditions for producing .alpha.-gypsum, its operation as the desulfurization process of exhaust gases is poor in stability and moreover, it is difficult to produce both .alpha.-gypsum and gypsum dihydrate.
On the other hand, as the method disclosed in Japanese Patent Application Laid-Open (KOKAI) No. 55-162,426/1980 is the method to obtain gypsum via gypsum dihydrate without using an organic acid, it does not have the above-mentioned inconvenience, however, since any catalyst agent for crystallization of carboxylic acid-series is not used in this method, .alpha.-gypsum of small in the aspect ratio cannot be formed.
Accordingly, a process wherein sulfur dioxide in an exhaust gas is fixed as gypsum dihydrate and then the gypsum dihydrate is converted to .alpha.-gypsum, and the medium containing a catalyst for crystallization of carboxylic acids is used in recirculation in the desulfurization system, has not yet been proposed.
Because such a situation is based on the facts that (1) the composition of the absorbent liquid for desulfurization of the exhaust gas is substantially independent to those of the medium for production of .alpha.-gypsum, and in spite of (1), (2) these two liquids circulate in the two processes (desulfurization and production of .alpha.-gypsum) resulting in the difficulty of preparing an aqueous liquid which acts effectively in both two processes.
On the basis of the findings, the present invention has been attained.